How to Size a Phase Converter — The Complete Guide

Choosing the right size rotary phase converter is critical to getting full performance from your three-phase equipment. An undersized converter can cause motors to overheat, trip breakers, or fail to start entirely — while an oversized unit wastes money. This guide walks you through the sizing process so you can make the right choice the first time.

Understanding Sizing Multipliers

Not every piece of equipment starts the same way. A table saw spins up easily compared to an air compressor, which must build pressure against a loaded piston. That's why phase converter sizing depends on the starting difficulty of your equipment:

• Easy-start loads (1.5× motor HP) — Equipment like saws, drill presses, and milling machines that spin up smoothly with minimal starting torque. These are the simplest loads to convert.
• Medium-start loads (1.75× motor HP) — Mixers and small lathes that need a moderate starting surge. They require a bit more headroom.
• Hard-start loads (2× motor HP) — Lathes, CNC machines, pumps, press brakes, and wide-belt sanders draw significant inrush current when starting. Sizing at twice the motor HP provides a reliable safety margin.
• Very hard-start loads (2.5× motor HP) — Air compressors, HVAC units, fans, and hydraulic presses are the most demanding. They start under heavy load and require 2.5 times the motor HP to ensure reliable operation.
• Resistive loads (1 HP per amp) — Welders, battery chargers, and ovens draw current differently. Size these at 1 HP of converter capacity for each amp of draw.

After applying the multiplier, round up to the next standard converter size. Our GP-NL series comes in 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50, 60, 75, and 100 HP models — so there's always a size that fits your application perfectly.

Running Multiple Machines Simultaneously

If you're running more than one three-phase machine at the same time, you need a converter sized for the combined total HP of all machines that may operate simultaneously. Add up the horsepower of every machine that could be running at once, apply the highest applicable multiplier, and round up. When in doubt, size up — a slightly larger converter runs cooler and lasts longer.

Checking Your Electrical Service

Your converter needs adequate single-phase power to generate three-phase output. A general rule: your panel should provide at least 2.5× the converter's rated HP in KVA. For a 200-amp, 240V single-phase service, you have roughly 48 KVA available — enough for converters up to about 20 HP, assuming no other major loads. If your calculation suggests a converter that might strain your service, consider upgrading your panel or calling us for a custom assessment.

Why Phoenix Phase Converters?

Every GP-NL phase converter is Built in the USA with industrial-grade components designed for continuous duty. Our proprietary designs deliver balanced voltage across all three phases, which means your motors run cooler, quieter, and last longer. We include a full manufacturer's warranty, and our sizing experts are available at 800-417-6568 to help you get exactly the right unit.

Common Equipment Sizing Reference Table

Frequently Asked Questions

Phase Converter Applications

We build phase converters for every industry. Explore our application-specific guides for sizing tips, wiring diagrams, and recommendations tailored to your trade:

Still not sure what size you need? Our experts have sized thousands of phase converters for shops, farms, and factories across the country. Call 800-417-6568 for a free consultation — we'll get you the right converter, guaranteed.